The present application relates to electronic lighting. More specifically, it relates to producing a low glow current to pre-heat lamp cathodes in a voltage fed electronic ballast. It is to be understood, however, that the present application can be applied to other lighting applications and ballasts, and is not limited to the aforementioned application.
Typical programmed start ballasts provide a low-glow preheating current to an attached lamp when the ballast is activated. This preheating extends the life of the lamp because it helps to avoid damage to the cathodes of the lamp that would accompany firing the lamp with cold cathodes. Typically, before striking the lamp, a ballast would enter a preheat mode controlled by an integrated circuit (IC), usually a high voltage IC. This IC could drive the inverter above and below resonance, and resultantly, it would require capacitive mode detection to avoid damage to the MOSFET switches of the inverter. If the intrinsic diodes of the MOSFETs turns conductive before gate turnoff, the MOSFET could be damaged or destroyed. Capacitive mode detection helps to prevent this.
As an alternative to an IC controller, a self-oscillating mode with inverter clamping has been used. This alternative tends to shorten lamp life because the pre-heat glow current is too high. Presently there is no reliable way to provide a low current preheat signal in a non-capacitive mode.
The present application contemplates a new and improved voltage fed electronic ballast that overcomes the above-referenced problems and others.